This application has been filed under 35 USC 171 as the United States stage of PCT/FR 92/00
1--In a cardiac pacemaker, pacing energy, e.g. ventricular, is applied to the wall of the ventricle by means of a pacing electrode in the form of a pulse having an amplitude representative of a voltage and a width representative of a durations.
The pacing energy is thus a function of a voltage and of a duration. Since this energy is stored in a battery implanted with the pacemaker, the life of the battery is directly related to the pacing energy.
A reduction of the pacing energy enables the size of the battery to be reduced and/or the life of the battery to be increased.
However, such a reduction of the pacing energy is to be carefully considered, since the stimulus must be efficient. In order to check the efficiency of a cardiac stimulus, the cardiac signal is analyzed as soon as possible after the stimulus so as to enable a counter-stimulus to be emitted if the stimulus has been inefficient. In order to get as near as possible to the stimulus, an analysis is made of the R-wave which has an amplitude in the region of 10 mV. Since the stimulus pacing signal has an amplitude of several volts, it is quite difficult to perceive the R-wave which is masked by residual electrical charges at heart-electrode interface level. To determine whether or not a stimulus has been efficient, it is necessary to know the influence of these electrical charges.
Furthermore, the minimum pacing energy entailing a cardiac response is defined as corresponding to the capture threshold. Various solutions have been proposed for the determination of capture threshold.
2. Description of the Prior Art
U.S. Pat. No. 3,920,024 describes a threshold test comprising a series of stimuli of decreasing energy with analysis of the endocardial signal between 20 and 40 ms after the stimulus. After an inefficient pacing pulse, successive counter-stimuli, spaced 50 ms apart and of increasing amplitude and width, are generated until cardiac response is obtained. After a threshold test, the pacing energy is recalculated from the energy of the first efficient counter-stimulus. In this patent, stimuli are generated by the distal electrode and detection is performed by the proximal electrode. There is therefore a change of pole between stimulus generation and detection.
European Patent No. 0,372,698 describes a pacemaker comprising a switched-capacitor amplifier with selective changing of the bandwidth and gain between the detection of the spontaneous complexes and the verification of capture. The gain is increased and the low cut-off frequency is reduced to verify capture, e g. by detection of the T-wave. T-wave detection is tardy and does not enable a counter-stimulus to be generated.